Catalytic process for preparing olefins by hydrocarbon pyrolysis

ABSTRACT

A process and catalyst for the catalytic pyrolysis of hydrocarbon to olefins and diolefins at temperatures in the range 600°-750° C. and pressures of 0.1 to 20 atmospheres. The catalyst is a bifunctional synthetic modified mordenite zeolite of the formula (yH·zM·uNa) O·Al 2  O 3  ·SiO 2  wherein M is Cu, Ag or Co/2 and u+y+z approach or equal 2. Good yields of ethylene and propylene are obtained from hydrocarbon feedstocks having boiling points up to 550° C.

FIELD OF THE INVENTION

This invention relates to a catalytic process for obtaining lower olefins by hydrocarbon pyrolysis using a specific bifunctional catalyst.

BACKGROUND OF THE INVENTION

Ethylene and propylene, butenes, butadiene and isoprene are obtained, at present, by the pyrolysis, in the presence of water vapor, of lower paraffins (ethane, propane, butane) or of low boiling hydrocarbon cuts such as straight run gasoline with reduced content of aromatic hydrocarbons.

The shortage of these raw materials in the past few years has made it desirable to use the heavy cuts of hydrocarbons, namely gas oils, vacuum distillates or crude oil as the pyrolysis feedstock.

As shown in Table 1, depending on the oil cut used, the yields obtained are different. (M.I. Offer, Hydrocarbon Processing 55 No. 10, Oct. 1976, p. 123).

                  TABLE 1                                                          ______________________________________                                         Distribution of products from cracking of the                                  hydrocarbons cuts with water vapor.                                            Cut                                                                            Distilling range                                                               °C. Gasoline  Gas oil     Gas oil                                       Pyrolysis  32-171    DA 232-327  DAV 343-546                                   Severity   Low    High   Low     High                                                                               Low   High                                ______________________________________                                         CH.sub.4   10.3   15.0   8.0   13.7  6.0   9.1                                 C.sub.2 H.sub.4                                                                           25.8   31.3   19.5  26.0  16.0  19.0                                C.sub.2 H.sub.6                                                                           3.3    3.4    3.3   3.0   3.2   3.8                                 C.sub.3 H.sub.6                                                                           16.0   12.1   14.0  9.0   12.5  12.2                                1.3-C.sub.4 H.sub.6                                                                       4.5    4.2    4.5   4.2   4.5   4.9                                 Other C.sub.4 hydro-                                                           carbons    7.9    2.8    6.4   2.0   5.4   3.4                                 C.sub.5 cut-205° C.                                                                27.0   22.0   20.0  20.0  14.0  11.0                                Fuel Oil   3.0    6.0    21.0  19.0  31.0  32.0                                Other components                                                                          2.2    3.2    3.3   3.1   7.0   4.6                                 ______________________________________                                    

Crude oil cracking in the presence of water vapor at very high temperatures (Japan Pat. No. 48-10763.1973) was recently tested on an industrial scale. The results obtained are given in Table 2.

                  TABLE 2                                                          ______________________________________                                         Crude oil pyrolysis by Kureha-Union                                            Carbide process.                                                                             Crude oil from                                                   Feed            Syria   Minas   Aramco Kafuji                                    0             1       2       3      4                                       ______________________________________                                         Specific weight                                                                (g/cm.sup.2)    0.840   0.844   0.853  0.880                                   Gasoline up to 200°                                                                     40      16      32     27                                      Oil (220°-250° C.) % wt.                                                         20      8       12     8                                       Gas oil (250°-350° C.) % wt.                                                     15      10      10     10                                      Waste oil % wt. 25      66      46     55                                      Sulphur         0.07    0.07    1.72   2.92                                    Working conditions:                                                            Steam temperature (° C.)                                                                1650    1450    2000   1650                                    Gravimetric ratio of                                                           water to crude oil                                                                             2.6     3.2     2.8    2.6                                     Reaction time                                                                  (seconds)       0.01    0.01    0.005  0.01                                    Products (% wt.)                                                               CH.sub.4        7.7     5.1     13.7   7.8                                     C.sub.2 H.sub.2 1.4     1.1     17.7   1.6                                     C.sub.2 H.sub.4 26.2    29.4    18.6   25.2                                    C.sub.3 H.sub.6 9.5     11.2    0.7    8.7                                     C.sub.4 H.sub.6 3.4     3.7     0.5    2.8                                     H.sub.2 + C.sub.3 -C.sub.4 cut                                                                 12.8    5.5     8.8    7.9                                     Benzene cut; boiling temp.                                                     cut 200° C.                                                                             2       1       5      1                                       Naphthene cut; boiling temp.                                                   cut 200°-250° C.                                                                 12      11      4      10                                      Heavy cut 250°-450° C.                                                           15      20      6      15                                      Waste cut; boiling temp.                                                       cut > 450° C.                                                                           10      12      25     20                                      ______________________________________                                    

The disadvantages of vapor cracking of heavy crude oil cuts are mainly the large amount of waste fuel and from the relatively low yields of C₂ -C₄ olefins. Improvements in this field can be obtained if the process is performed in the presence of thermal carriers (coke, silica, refractary oxides) preferably in a fluidized bed or mobile bed of a thermal carrier. A synthesis of these processes is described by R. Chelle and G. Henrich (Chimie et Industrie, Genie Chimique Vol. 104, No. 4-5 March 1971, p. 413-421).

Up to the present these processes have not been developed on industrial scale due to the fact they require high reaction temperatures within the operation range of cracking units with water vapor, the mechanical corrosion of reactors is an important deterrent.

Recently, starting from these processes, the catalytic processes of hydrocarbon pyrolysis have been tried.

According to West German Pat. No. 1,927,061, the pyrolysis reaction is achieved on catalysts consisting of Mg and Zr oxide mixtures, with lanthanide admixtures, Fe₂ O₃, Al₂ O₃, SiO₂ or Ca, Mg, Sr and Ba oxides. Alternative catalysts based on refractory metal oxide are described in the following patents: West German Pat. No. 1,971,400, England Pat. No. 1,369,242, France Pat. No. 1,603,019, U.S. Pat. Nos. 3,725,495 and 3,767,567, Japan Pat. No. 7,504,002. Also according to the Soviet Union Pat. No. 410,073, the use of potassium niobiate as a catalyst has the advantage of reducing the coke evolved in the process. At present niobium is not available in large amounts for such use.

All these catalysts have the disadvantage that they work at high temperatures, up to 900° C., within the range of thermal processes.

Hence it is difficult to emphasize the catalytic contribution to the pyrolysis process and to estimate the difference between these processes and the process with thermal carriers.

A catalyst working at relatively low temperatures (500°-700° C.) consisting of fluorinated alumina is claimed in the West German Pat. No. 1,277,842. Hydrocarbon cuts from C₄ to cuts having boiling temperatures of 450° C. can be used as the pyrolysis feedstock.

It is a specific object of this process to obtain propylene as the main product, the ratio of propylene to ethylene being higher than 1 and it provides a high content of aromatic hydrocarbons in the liquid cut from the pyrolysis. The low ethylene yields cannot be improved by recycling the liquid pyrolysis cut due to its aromatic nature. Fluorine also causes instability and corrosion in the apparatus.

The use of new types of catalysts, completely cationized molecular sieves with a ratio of SiO₂ to Al₂ O₃ within the range 3 and 20 is described in the U.S. Pat. No. 3,647,682. The relative low thermal stability of molecular sieves makes it possible to use them at relatively low temperatures and to obtain mainly propylene and saturated C₂ and C₃ hydrocarbons.

THE INVENTION

The catalytic process for preparing lower olefins according to the present invention eliminates the disadvantages of the known processes by using a specific bifunctional catalyst based on a mordenite type synthetic aluminosilicate with ratios of SiO₂ to Al₂ O₃ higher than 20, in accordance with the formula: (yH.zM.uNa.) O.Al₂ O₃.nSiO₂ : where y+z+U≦2 M is Cu, Ag or Co/2, n is a number more than 20 and preferably about 25.

To this may be added v RO_(x) by adsorption or co-crystallization: R is lead or cerium rare earth or other metal having a large ion radius, v is 0.005 to 0.5 wt. % of the mordenite zeolite aluminosilicate. In general, catalysts where z is 0.3 and y is 1.6 are preferred. By modifying the basic catalyst with metal oxides having a large ionic radius such as Ce, Pb and rare earth oxides, the amount of coke deposition on the catalyst is reduced without any essential change of the catalytic properties. The modifying oxides have the formula v RO_(x) where v=0.005 to 0.5 wt. % of said aluminosilicate catalyst and x is a value depending upon the valence of the metal R.

The high activity peculiar to this catalyst as well as its high thermal stability makes it possible to use it at temperatures of 600°-750° C., well within the temperatures commonly used for both the catalytic cracking and pyrolysis processes. This makes possible the use in reactor design of the rich experience previously acquired from these processes.

Another characteristic of the process is its high flexibility. It is capable of being adjusted to accommodate various raw materials from butane to oil cuts with boiling temperatures above 500° C. Lower olefins and diolefins yields are high and the ratio of the various olefins can be modified within wide limits. The liquid cut consists mainly of gasolines with the boiling range of 50°-205° C., even when heavy oil cuts are used as feedstocks.

Reduced amounts of acetylene, aromatic and isoparaffin hydrocarbons are obtained in this process, thus simplifying the further separation and purification stages of the reaction products and giving the possibility of recycling the liquid cuts. The consumption of raw material/ton of ethylene can thus be reduced to 2 tons raw material/ton of ethylene. It is surprising that the pressures, up to 20 atmospheres, and the absence of diluents, such as water vapor, does not worsen process performances leading to a further simplification of separation units and to a decrease of the power consumption necessary to compress the resultant gases.

Due to coke deposition the process of this invention is cyclic. It can be carried out in all known types of reactors, such as having fixed, mobile or fluidized catalyst beds.

The catalyst is periodically regenerated by controlled burning of the accumulated coke with air or inert gases containing oxygen.

The catalyst may be premixed with heat carriers such as are used in thermal pyrolysis. Among such carriers, corundum (α alumina) is useful and does not interfere with the catalysis or the catalysts of this invention.

Hereinafter are given examples to illustrate but not to limit the procedure.

EXAMPLE 1

Mordenite having the formula (1.9H.0.1Na)O.Al₂ O₃.25 SiO₂, prepared and decationized by known processes, is modified by ion exchange with Cu, Ag or Co and molded as extrudates with diameter of 1-2 mm. The catalyst is fed into a catalytic reactor of 100 cm³ capacity. Normal butane is steadily passed at a volume speed equal to 750 h⁻¹ over the catalyst heated at 650° C.

The distribution of reaction products obtained is shown in Table 3 and compared to the thermal process without catalyst but in the presence of the heat carrier consisting of α-alumina, corundum type, with high thermal conductivity, and of size close to the catalyst sizes.

Under identical working conditions, but in the absence of the thermal carrier, the conversion degree of n-butane is reduced to half.

The comparison data obtained with the monofunctional catalyst No. 1 (consisting of decationized mordenite), and catalysts 2, 3 and 4 shows the higher activity of the bifunctional catalysts of this invention.

                  TABLE 3                                                          ______________________________________                                         n-Butane pyrolysis                                                                                                  Thermal                                   Catalyst Components                  process with                              (% wt)       1      2      3    4    heat carrier                              ______________________________________                                         H.sub.2      0.10   0.05   0.06 0.04 0.08                                      CH.sub.4     4.90   8.90   8.60 12.70                                                                               5.10                                      C.sub.2 H.sub.4                                                                             8.20   11.40  10.40                                                                               10.50                                                                               4.20                                      C.sub.2 H.sub.6                                                                             5.50   7.60   7.80 14.1 3.80                                      C.sub.3 H.sub.6                                                                             11.00  12.40  12.80                                                                               13.5 8.70                                      C.sub.3 H.sub.8                                                                             2.20   7.30   5.30 9.1  0.10                                      C.sub.4 H.sub.8 + C.sub.4 H.sub.6                                                           1.70   4.10   4.4  5.3  1.1                                       C.sub.4 H.sub.10                                                                            66.40  48.25  50.64                                                                               34.76                                                                               76.92                                     ______________________________________                                          1=(1.9H . O . 1Na) . O . Al.sub.2 O.sub.3 . 25SiO.sub.2                        2=(1.6H . 0.3Cu. O . 1Na) . O . Al.sub.2 O.sub.3 . 25SiO.sub.2                 3=(1.6H . 0.3Ag . 0.1Na) . O . Al.sub.2 O.sub.3 . 25SiO.sub.2                  4=(1.6H . 0.3Co . 0.1Na) . O . Al.sub.2 O.sub.3 . 25SiO.sub.2            

EXAMPLE 2

Mix 250 cm³ catalyst 3 from Table 3 with 250 cm³ α-alumina and fill a tube reactor of 500 cm³ reaction volume.

Over the catalyst, heated to the medium temperature of 725° C., pass for 2 h. 750 gr n-paraffins C₉ -C₁₄ and 1500 gr water vapor at atmospheric pressure. Initially, the pressure drop in the catalyst bed is 0.28 atmos. At the end of the experiment it amounts to 0.30 atmos. due to coke deposition. Table 4 shows the distribution of the products obtained.

                  Table 4                                                          ______________________________________                                         Pyrolysis of n-paraffins on catalyst                                           no. 3 (Table 3)                                                                              n-paraffins C.sub.9 -C.sub.14                                                                  99.1% wt                                                       isoparaffins     0.4% wt                                                       aromatics        0.5% wt                                         Raw material  sulphur (ppm)   38                                                characteristics:                                                                             ##STR1##         0.7496                                         ______________________________________                                         Product distribution                                                           (% wt.)                                                                        H.sub.2       0.87                                                             CH.sub.4      7.93                                                             C.sub.2 H.sub.6                                                                              2.48                                                             C.sub.2 H.sub.4                                                                              42.13                                                            C.sub.3 H.sub.8                                                                              0.67                                                             C.sub.3 H.sub.6                                                                              18.79                                                            C.sub.4 H.sub.10                                                                             0.12                                                             C.sub.4 H.sub.8                                                                              6.79                                                             1.3C.sub.4 H.sub.6                                                                           5.91                                                             C.sub.5 H.sub.12                                                                             0.05                                                             C.sub.5 H.sub.10                                                                             0.77                                                             1.3 iso-C.sub.5 H.sub.8                                                                      1.09                                                             F--C.sub.6 cut                                                                               2.47                                                             n-paraffin C.sub.7 --C.sub.14                                                                6.29                                                             iso-paraffin -olefins                                                                        1.82                                                             aromatics     1.54                                                             carbon oxide (% vol.)                                                                        0.28                                                             acetylene (ppm)                                                                              600                                                              ______________________________________                                    

The amount of coke deposition on catalyst is 0.4%. Catalyst regeneration is achieved by passing an air flow of 300 l/h for 2 hours at 600° C.

EXAMPLE 3

The catalyst in example 2 is modified by deposition thereon of cerium from cerium sulphate. The proportion of is 0.1% wt. of cerium referred to total catalyst weight. Under the conditions given in example 2, the content of coke deposited is 0.2%. Equivalent results are obtained by modifying the catalyst with Pb.

EXAMPLE 4

250 cm³ catalyst with the formula:

    (1.6H.03 Co/2.0.1Na).O.Al.sub.2 O.sub.3.25SiO.sub.2 +0.05 Ce.sub.2 O.sub.3

as extrudates with a diameter equal to 2 mm is mixed with 250 cm³ α-alumina and fed into a 500 cm³ reactor. A mixture of water vapors and hydrocarbons is passed over the catalyst heated at medium temperature of 720° C.

Table 5 shows the working conditions, the characteristics of the raw materials and the distribution of the reaction products.

                                      TABLE 5                                      __________________________________________________________________________     The influence of raw material                                                  composition.                                                                   __________________________________________________________________________     Raw material                                                                              1. Heavy gasoline                                                                          2. Gas oil Da                                                                            3. DAV gas oil                                Distilling curve                                                                          Initial:                                                                             148   200       Initial: 285° C.                       °C. 10% Vol.                                                                             150   249       Final: 514° C.                                    20% Vol.                                                                             162   260       Hydrofined                                               30% Vol.                                                                             164   267       at medium                                                40% Vol.                                                                             166.5 274       severity                                                 50% Vol.                                                                             168.5 282                                                                60% Vol.                                                                             170   292                                                                80% Vol.                                                                             173.5 315       Characteristics                                          90% Vol.                                                                             177.5 338       after hydrofinin                                         Final 208 (95%)                                                     Density (D.sub.4.sup.20)                                                                  0.781       0.8600    0.8468                                        Mean molecular                                                                 mass       131         230       259                                           Composition (% Vol.)                                                                      Paraffins: 41.0                                                                            Paraffins+                                                                               Paraffins 55% vol.                                       Naphthenes 44.5                                                                            naphthenes                                                                               Naphthenes: 27.5%                                                    70% vol.  vol.                                                                 Olefins 2%                                                         Aromatics: 14.5                                                                            Aromatics: 28% Vol.                                                                      Aromatics:                                                                     17.5 vol.                                                Sulphur ppm 340                                                                            1800      800                                                      Working conditions                                                             Hydrocarbons feed                                                              (hours.sup.-1) 2.6                                                                         3.8       2.6                                                      Ratio of steam to                                                              hydrocarbon 0.97                                                                           1.18      0.90                                                     Product distribution                                                H.sub.2    0.86        0.81      0.60                                          CH.sub.4   10.24       6.15      11.57                                         C.sub.2 H.sub.6                                                                           2.36        2.51      6.80                                          Raw material                                                                              1. Heavy gasoline                                                                          2. Gas Oil DA                                                                            3. DAV gas oil                                C.sub.2 H.sub.4                                                                           27.50       17.01     16.90                                         C.sub.3 H.sub.8                                                                           0.52        0.56      1.04                                          C.sub.3 H.sub.6                                                                           15.49       13.59     16.14                                         C.sub.4 H.sub.10                                                                          0.31        0.12      0.16                                          C.sub.4 H.sub.8                                                                           7.19        7.28      6.68                                          C.sub.4 H.sub.6                                                                           4.68        2.78      3.07                                          C.sub.5 H.sub.12                                                                          0.19        0.42      0.12                                          C.sub.5 H.sub.10                                                                          2.63        4.50      1.30                                          C.sub.5 H.sub.8                                                                           2.50        2.27      1.10                                          Gasoline                                                                       60°-205° C.                                                                 25.52       20.50     19.77                                         Heavy cut  0           21.50     14.74                                         205°-320° C.                                                     Acetylene (ppm)                                                                           850         700       780                                           CO (% vol.)                                                                               0.91        0.85      0.90                                          C.sub.3 /C.sub.2                                                                          0.56        0.79      0.96                                          weight                                                                         TOTAL olefins                                                                  C.sub.2 --C.sub.5                                                                         59.99       47.43     45.19                                         __________________________________________________________________________

EXAMPLE 5

In order to emphasize the influence of the diluent (water vapors) on the catalyst and unit of Example 2, experiments were carried in the presence and absence of water vapor. The results obtained are given in Table 6.

                  TABLE 6                                                          ______________________________________                                          The influence of raw material dilution with                                   water vapors.                                                                           Heavy gasoline  n-Paraffins C.sub.9 --C.sub.14                                 (characteristics                                                                               (Characteristics of                                   Raw material                                                                            of table 5)     table 4)                                              Working                                                                        conditions                                                                              1      2      3    4    5    6    7    8                              ______________________________________                                         Water (1/1                                                                     cat. hour)                                                                              0      0      0    1.3  0    0    0    2.9                            Hydrocarbon                                                                    (1/1 cat.h)                                                                             4.0    5.2    7.5  2.6  3.6  5.6  5.6  2.8                            Temperature                                                                    °C.                                                                              690    690    690  700  715  690  680  700                            Distribution                                                                   of reaction                                                                    products                                                                       H.sub.2  1.05   0.54   0.48 0.70 3.52 1.37 0.47 1.02                           CH.sub.4 14.11  9.62   8.57 8.14 14.84                                                                               11.75                                                                               6.65 6.75                           C.sub.2 H.sub.6                                                                         5.75   5.00   4.48 3.10 11.61                                                                               10.62                                                                               7.24 2.44                           C.sub.2 H.sub.4                                                                         21.25  17.81  15.64                                                                               17.27                                                                               26.00                                                                               26.91                                                                               22.48                                                                               19.47                          C.sub.3 H.sub.8                                                                         0.93   0.88   0.77 0.55 1.68 2.07 1.65 0.65                           C.sub.3 H.sub.6                                                                         17.35  18.17  15.06                                                                               13.24                                                                               20.20                                                                               22.13                                                                               20.31                                                                               13.16                          C.sub.4 H.sub.10                                                                        0.24   0.23   0.19 0.42 0.39 0.56 0.41 0.23                           C.sub.4 H.sub.8                                                                         6.17   6.29   5.50 4.60 6.50 6.86 6.01 7.48                           C.sub.4 H.sub.6                                                                         4.82   4.75   4.63 4.35 5.86 4.52 4.81 5.39                           C.sub.5 H.sub.12                                                                        0.05   0.04   0.03 0.48 0.16 0.21 0.39 0.22                           C.sub.5 H.sub.10                                                                        3.34   3.03   2.60 3.50 4.75 2.36 3.10 4.10                           C.sub.5 H.sub.8                                                                         2.04   2.03   1.98 2.13 3.29 3.02 2.77 2.08                           Gasoline                                                                       60°-205° C.                                                               22.89  31.61  40.52                                                                               41.52                                                                               1.20 7.62 23.70                                                                               27.01                          C.sub.3 /C.sub.2                                                                        0.82   1.02   0.96 0.77 0.78 0.82 0.90 0.68                           (weight)                                                                       Total olefins                                                                           54.97  52.08  45.41                                                                               45.09                                                                               66.60                                                                               65.80                                                                               59.48                                                                               51.68                          C.sub.2 --C.sub.5                                                              ______________________________________                                    

Example 6

In order to emphasize pressure influence, experiments were carried on the catalyst and in the unit of Example 2 at different pressures with two types of raw materials.

Table 7 shows the results.

                  TABLE 7                                                          ______________________________________                                         Pressure influence.                                                                         n-Paraffins C.sub.9 -C.sub.14                                                                DAV gas oil                                                      (characteristics                                                                             (Raw material 3                                     Raw material of Table 4)   of Table 5)                                         ______________________________________                                         Working condi-                                                                 tions        1      2      3     1    2    3                                   Hydrocarbons                                                                   (hours.sup.-1)                                                                              2.8    2.8    5.6   2.4  3.6  2.4                                 H.sub.2 O:HC(wt)                                                                            1.48   1.45   1.48  0.90 0.75 0.90                                Temperature                                                                    (°C.) 690    690    690   690  675  690                                 Outlet pressure                                                                (ate)        5      10     10    3.5  10.5 17.5                                Distribution of                                                                products                                                                       (% wt)                                                                         H.sub.2      1.19   1.08   0.58  0.31 0.26 0.97                                CH.sub.4     13.68  18.43  10.49 13.79                                                                               10.13                                                                               17.94                               C.sub.2 H.sub.6                                                                             9.58   14.41  8.40  7.21 6.98 11.10                               C.sub.2 H.sub.4                                                                             31.07  33.84  26.06 12.65                                                                               10.75                                                                               14.82                               C.sub.3 H.sub.6                                                                             1.19   1.08   0.77  1.23 1.37 1.31                                C.sub.3 H.sub.8                                                                             17.54  16.84  12.72 12.07                                                                               9.61 11.58                               C.sub.4 cut  13.10  6.33   5.19  4.63 4.24 2.91                                Gasoline                                                                       30°-205° C.                                                                   12.65  7.99   38.26 28.01                                                                               34.52                                                                               20.16                               Cut 205°-360° C.                                                              0      0      0     20.10                                                                               22.14                                                                               19.21                               C.sub.3 -C.sub.2 (wt)                                                                       0.56   0.49   0.48  0.95 1.06 0.78                                ______________________________________                                    

EXAMPLE 7

In order to emphasize temperature influence, experiments were carried on the catalyst and raw material of Example 2, at different temperatures. Results are given in Table 8.

                  TABLE 8                                                          ______________________________________                                         Temperature influence.                                                         Working conditions                                                                              1       2       3     4                                       ______________________________________                                         Hydrocarbon (hours.sup.-1)                                                                      2.8     2.8     2.8   3.0                                     H.sub.2 O: Hydrocarbon                                                         (weight)         3.0     2.9     2.8   2.0                                     Temperature (°C.)                                                                        650     680     705   725                                     Distribution of                                                                products (% wt)                                                                H.sub.2          0.94    1.05    1.21  0.81                                    CH.sub.4         1.16    2.93    6.65  7.93                                    C.sub.2 H.sub.6  0.62    1.21    2.61  2.87                                    C.sub.2 H.sub.4  4.85    10.09   21.97 41.90                                   C.sub.3 H.sub.6  11.18   12.69   14.98 17.38                                   C.sub.3 H.sub.8  0.63    0.42    0.71  0.87                                    C.sub.4 cut      9.46    11.54   14.11 12.81                                   C.sub.5 cut      4.38    5.67    5.12  4.50                                    Cut 60°-205° C.                                                                   66.78   54.40   32.64 10.93                                   C.sub.3 : C.sub.2 (wt)                                                                          2.30    1.26    0.68  0.41                                    TOTAL olefins                                                                  C.sub.2 --C.sub.5                                                                               29.87   39.99   56.18 76.59                                   ______________________________________                                     

I claim:
 1. A process for preparing lower olefins and diolefins of 2 to 5 carbon atoms by hydrocarbon pyrolysis wherein a hydrocarbon feedstock is contacted with a catalyst at a temperature in the range of 600°-750° C. at pressures in the range of 0.1 to 20 atmospheres, said catalyst being a bifunctional catalyst based upon a modified mordenite zeolite aluminosilicate having the formula:

    (yH.zM.uNa)O.Al.sub.2 O.sub.3.nSiO.sub.2

where M is selected from the group consisting of Cu, Ag, or Co/2 y, z and u are all greater than zero, y+z+u equals or approximates 2, and n is more than 20, and said catalyst additionally contains from 0.005 to 0.5% by weight of said catalyst of an oxide of a metal selected from the group of lead and cerium oxides.
 2. The process according to claim 1 wherein said catalyst is premixed with a heat carrier for thermal pyrolysis.
 3. The process according to claim 2 wherein said heat carrier is α alumina.
 4. The process according to claim 1 wherein said feedstock additionally contains water as a diluent.
 5. The process according to claim 1 wherein said pressure is in the range of 1 to 10 atmospheres.
 6. The process according to claim 1 wherein said temperature is in the range 650° to 725° C.
 7. The process according to claim 1 wherein said hydrocarbon feedstock is selected from the group of petroleum hydrocarbons ranging from gasoline to heavy oil cuts.
 8. The process according to claim 7 wherein its hydrocarbons in said feedstock are mixed hydrocarbon cuts having boiling points in the range 50° to 550° C.
 9. The process according to claim 1 wherein said contacting step with said catalyst is in a fixed catalyst bed.
 10. The process according to claim 1 wherein said contacting step with said catalyst is in a mobile catalyst bed.
 11. The process according to claim 1 wherein said contacting step with said catalyst in a fluid catalyst bed.
 12. The process according to claim 1 wherein periodically after continuous operation said contacting of said feedstock is discontinued and the catalyst is regenerated by burning off any coke accumulated on said catalyst by the passage of an oxygen-containing gas through the hot catalyst. 